1. Field of the Invention
The present invention relates to a driving force distributing apparatus which is applied to a four-wheel drive vehicle and which distributes a driving force from a prime mover to front wheels and rear wheels in predetermined distributing proportions and particularly to a driving force distributing apparatus in which the number of parts is reduced to cut a cost and a capacity is reduced to lessen constraints to a vehicle design without impairing excellent responsivity of a prior-art apparatus.
2. Description of the Related Art
In recent years, as use of automobiles is diversified, more and more four-wheel drive vehicles are used suddenly. In the four-wheel drive vehicle, a driving force distributing apparatus for distributing a driving force generated by an engine to front wheels and rear wheels by a hydraulic multiple disc clutch is used conventionally.
FIG. 7 shows a basic structure of a four-wheel drive vehicle to which the prior-art driving force distributing apparatus is applied. The driving force generated by an engine 101 is transferred to the driving force distributing apparatus 1 through a transmission 102 and a center drive shaft 103, hydraulic oil at predetermined hydraulic pressure is supplied from a hydraulic pressure unit 5 to a multiple disc clutch 23 built into the driving force distributing apparatus 1, and the driving force from the engine 101 is transferred to rear wheels 105 through the center drive shaft 103 and a differential 104 and is transferred to front wheels 107 through a front drive shaft 106 and the differential 104 in predetermined distributing proportions. Control of hydraulic pressure to the multiple disc clutch 23 is carried out by controlling the hydraulic pressure unit 5 by the control circuit 6.
FIG. 8 shows the prior-art driving force distributing apparatus 1 shown in FIG. 7. The driving force distributing apparatus 1 includes an apparatus main body 2A, the hydraulic pressure unit 5 connected to the apparatus main body 2A through a pipe 5a, and the control circuit 6 for controlling the hydraulic pressure unit 5. In the apparatus main body 2A, the multiple disc clutch 23, a pressing member 25 for pressing the multiple disc clutch 23, a lever 13 for pressing the pressing member 25, and a piston 14 for moving the lever 13 in a rotating manner are disposed.
The hydraulic pressure unit 5 includes an oil pump 50 for pressure-sending the hydraulic oil, a pump motor 51 for driving the oil pump 50, an accumulator 53 for accumulating a pressure of the hydraulic oil pressure-sent from the oil pump 50 through a check valve 52, a pressure switch 54 for detecting line pressure, a pressure control valve 56 connected through a filter 55 to control the line pressure, a relief valve 57 for operating at pressure equal to or higher than predetermined pressure to relieve the hydraulic oil, and a reservoir tank 58 for receiving the hydraulic oil returning from the pressure control valve 56 and the relief valve 57.
In the driving force distributing apparatus 1 formed as described above, if the oil pump 50 pressure-sends the hydraulic oil by driving of the pump motor 51, the pressure of the hydraulic oil is accumulated by the accumulator 53 and the pump motor 51 is ON/OFF controlled by the pressure switch 54 such that the line pressure is in a predetermined pressure range. A part of the hydraulic oil pressure-sent from the oil pump 50 and controlled to be at pressure in the predetermined pressure range is returned to the reservoir tank 58 according to an opening degree of the pressure control valve 56 controlled by the control circuit 6. As a result, hydraulic pressure of the hydraulic oil supplied to the piston 14, i.e., pressure on the multiple disc clutch 23 (clutch pressure) is adjusted. In this manner, by controlling the opening degree of the pressure control valve 56, the clutch pressure can be controlled further continuously and optimum power distribution can be achieved under various traveling conditions.
However, according to the prior-art driving force distributing apparatus, because many parts are necessary for a hydraulic pressure control mechanism and expensive parts such as a current control pressure regulating valve are included in the parts, the apparatus as a whole is expensive. Because a capacity of the hydraulic pressure unit is large, a place in the vehicle in which the hydraulic pressure unit is mounted is limited. Therefore, the hydraulic pressure unit needs to be disposed in a place at a distance from a transfer and the hydraulic piston on a transfer side and the hydraulic pressure unit need to be coupled by a flexible hydraulic pipe.
Therefore, it is an object of the invention to provide a driving force distributing apparatus in which the number of parts is reduced to cut a cost and a capacity is reduced to lessen constraints to a vehicle design without impairing excellent responsivity of the prior-art apparatus.
To achieve the above object, according to the present invention, there is provided a driving force distributing apparatus for pressing a multiple disc clutch with a predetermined pressing force by a pressing member to transfer a driving force from a prime mover to a front wheel and a rear wheel in distributing proportions corresponding to the predetermined pressing force, the apparatus comprising: a piston for pressing the multiple disc clutch through the pressing member; a pump for supplying pressure fluid to a piston chamber, the pressure fluid being for generating the pressing force for pressing the pressing member in the piston; a motor for driving the pump for normal rotation to supply the pressure fluid to the piston chamber; pressure fluid discharging means provided to a pressure fluid supply path extending from the pump to the piston chamber to discharge the pressure fluid on a piston chamber side in the pressure fluid supply path to an outside when a pressure difference between pressure on the piston chamber side and pressure on a pump side in the pressure fluid supply path becomes equal to or greater than a predetermined value; and control means for driving the motor for normal rotation to drive the pump for normal rotation in increasing pressure in the piston chamber and for driving the motor for reverse rotation to drive the pump for reverse rotation to make the pressure difference equal to or greater than the predetermined value in reducing the pressure in the piston chamber.
With the above structure, in reducing the pressure in the piston chamber, by driving the pump motor for reverse rotation, the pressure difference between the piston chamber side pressure and the pump side pressure in the pressure fluid supply path increases and the pressure fluid on the piston chamber side in the pressure fluid supply path is instantaneously discharged outside.